In recent years many electronic devices have been developed which emit or in someway modulate light. Because of these developments a need has arisen for a black coating which can be patterned by any of the several processes common in the microelectronics industry. This black coating may either be used to protect light sensitive areas of a device from light or to enhance contrast between light emitting or reflecting areas on a device and those areas which should appear darker. The polymer coating must do much more than merely absorb light. Black paint for example could not be used. The coating should be patternable preferrably by a process already used in the industry. One process involves coating positive photoresist exposing it to light and developing the photoresist and an underlying layer in hydroxide solution. This basic process is called a wet process or a wet etch. Another process involves coating resist on a substrate patterning the resist, etching the substrate and then etching a sublayer under the substrate using a plasma or a reactive ion etch. This is called a dry etch in the art. Another requirement of the coating is that the polymer coating should have dielectric properties. As a part of an electrical device it should have a high dielectric strength and a high resistivity. A third requirement is that the coating should be uniform. It should adhere well to common substrates such as silicon oxide and aluminum. The coating should be a strong absorber of light at the wavelengths of interest and it should high thermal stability and it should be durable. Also, the film thickness must be compatible with the thickness of the electrical device while still providing sufficient light attenuation.
In the art one material currently sold by Polytronics of Richardson, Tex. attempts to meet some of the above requirements but does not meet all of them. It contains a polymer solution filled with finely ground carbon. The polymer used is polyimide which imparts thermal stability but the material can be patterned only by a wet process, it cannot be dry etched. The material has extremely low resistivity and is thus not compatible with many electronic devices. Moreover the film is not uniform due to the carbon particles and light absorption per unit of film thickness is very low. The art has attempted to solve the problem of protecting light sensitive areas of a device in other ways including by depositing an insulating layer on the device and patterning a metal on top of the insulating layer. The metal shields portions of the device from light. However, this process is very time consuming and more expensive than using an opaque organic film. In addition many users do not like this system since they do not wish to reflect light because of affects on peripheral light sensitive devices or because the appearance of a very shiny surface is not satisfactory in their device. A further way of enhancing contrast in the art has been to deposit an anti-reflection layer such as an indium-tin-oxide on a part of the device which should appear optically darker. These layers are very limited in that they work only over a relatively narrow band of wavelengths and they require a more complex and longer process and are difficult to rework.
One final approach which the art has used has been to use a filter which is overlaid on the device that is, the filter is not deposited directly on the device. Due to the size and geometry of the filter and the complexity of the filter patterns these devices are limited due to problem of aligning the filter precisely with the underlying device.
Applicants' invention solves the problems of the prior art by using preferrably a polymer precursor and soluble dyes which form the uniform containing this film with extremely good insulating properties, good adhesion properties, wet and dry development characteristics with a photoresist system and a very high light absorbance in a desired wavelength range from ultraviolet through the visible and through the infrared spectrum. Applicants' coating material has a film thickness compatible with electronic and electrical devices and has a very high resolution of features compatible with such electrical and electronic devices. The light absorbing film demonstrates extremely good thermal, chemical and aging stability and may remain an integral part of an electrical or electronic device.